<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            height: 2000px;
        }
        .box1{
            width: 100px;
            height: 100px;
            margin-top: 100px;
            background-color: red;
        }
        .box2{
            width: 100px;
            height: 100px;
            margin-top: 100px;
            background-color: blue;
        }


    </style>
</head>

<body>
    <div class="box1">
    </div>
    <div class="box2">
    </div>

    <script>
        const io =new IntersectionObserver((entries)=>{
            console.log(entries);
        },{
            threshold:[0,0.25,0.5,0.75,1]

        })
        io.observe(document.querySelector('.box1'))


    </script>
</body>

</html>